Plant needing a drink (4)
I believe the answer is:
arum
'plant' is the definition.
(arum is a kind of plant)
'a drink' is the wordplay.
'drink' becomes 'rum' (rum is a kind of drink).
'a'+'rum'='ARUM'
'needing' acts as a link.
(Other definitions for arum that I've seen before include "Cuckoo pint genus" , "Plant like cuckoo pint" , "Cuckoo-pint" , "Type of 1 Across" , "Eg, cuckoo pint" .)